polyfill abort signal methods

src/platform/polyfills.ts

@@ -1,3 +1,76 @@
 import 'fast-text-encoding'
 
+/*
+ * React Native installs abort-controller@3 as its AbortController global. That
+ * package predates AbortSignal.reason and AbortSignal.throwIfAborted(), while
+ * newer libraries such as @atproto/lex-client expect the modern DOM API.
+ *
+ * Patch the shared prototype once so every signal - including signals created
+ * by dependencies - has browser-compatible abort behavior. The reason is
+ * stored before dispatching the abort event so listeners can read it
+ * synchronously, and the first abort reason wins as required by the DOM spec.
+ */
+const abortReasons = new WeakMap<AbortSignal, unknown>()
+const abortSignalPrototype = AbortSignal.prototype as AbortSignal & {
+  readonly reason?: unknown
+  throwIfAborted?: () => void
+}
+
+function createDefaultAbortReason(): unknown {
+  if (typeof DOMException !== 'undefined') {
+    return new DOMException('This operation was aborted', 'AbortError')
+  }
+  const error = new Error('This operation was aborted')
+  error.name = 'AbortError'
+  return error
+}
+
+if (!('reason' in abortSignalPrototype)) {
+  const originalAbort = AbortController.prototype.abort
+
+  Object.defineProperty(AbortController.prototype, 'abort', {
+    configurable: true,
+    writable: true,
+    value: function abort(this: AbortController, reason?: unknown) {
+      const signal = this.signal
+      if (!signal.aborted) {
+        abortReasons.set(
+          signal,
+          reason === undefined ? createDefaultAbortReason() : reason,
+        )
+      }
+      try {
+        originalAbort.call(this)
+      } catch (error) {
+        if (!signal.aborted) abortReasons.delete(signal)
+        throw error
+      }
+    },
+  })
+
+  Object.defineProperty(AbortSignal.prototype, 'reason', {
+    configurable: true,
+    get(this: AbortSignal) {
+      if (!this.aborted) return undefined
+      let reason = abortReasons.get(this)
+      if (reason === undefined) {
+        reason = createDefaultAbortReason()
+        abortReasons.set(this, reason)
+      }
+      return reason
+    },
+  })
+}
+
+if (typeof abortSignalPrototype.throwIfAborted !== 'function') {
+  Object.defineProperty(AbortSignal.prototype, 'throwIfAborted', {
+    configurable: true,
+    writable: true,
+    value: function throwIfAborted(this: AbortSignal) {
+      if (!this.aborted) return
+      throw (this as AbortSignal & {readonly reason: unknown}).reason
+    },
+  })
+}
+
 export {}